Currently, with promotion of the 3rd Generation (3rd Generation, 3G) mobile communication technology, Time Division-Synchronous Code Division Multiple Access (Time Division-Synchronous Code Division Multiple Access, TD-SCDMA) networks are operated. The TD-SCDMA not only requires frequency synchronization required by general cellular communication, but also requires time synchronization that is accurate to within ±1 us end to end. Meanwhile, the future 3G Long Term Evolution (Long Term Evolution, LTE) and Worldwide Interoperability for Microwave Access (Worldwide Interoperability for Microwave Access, Wimax) will also use the same time division duplexing (Time Division Duplexing, TDD) mode as the TD-SCDMA, and will also require phase and time synchronization. Therefore, it is urgently necessary to use the existing bearer network to implement precision time transfer.
A universal time transfer scheme in bearer networks is the 1588 protocol. Time synchronization and transparent transmission may be implemented by the 1588 protocol, and it is regarded as universal time transfer solution. The 1588 protocol calculates a path delay and an absolute time deviation between a master device and a slave device through a timestamp generated by exchanging time packets between the master device and the slave device, thereby accomplishing time synchronization between the master device and the slave device.
A prerequisite of using the 1588 protocol to implement network time synchronization is that the path delay in the receiving direction is the same as the path delay in the transmitting direction. In a practical network, however, the path delays in both directions are usually different, and it will cause the error of the absolute time deviation calculated through the 1588 protocol. The time precision required by the mobile network is not fulfilled unless the path delays in both directions are measured accurately and a time compensation is made properly.
Currently, a method for measuring the path delays in both directions is to use an optical time domain reflectometer (Optical Time Domain Reflectometer, OTDR) to measure the fiber length of the receiving line and the fiber length of the transmitting line between devices manually, calculate a time synchronization error, and correct the time synchronized by the 1588 protocol.
Such a method of accomplishing time synchronization by using the OTDR to measure and calculate the time synchronization error of the 1588 protocol and compensate for the time manually is complicated and inaccurate because fiber lengths of the lines in both directions needs to be measured between every two devices when the transmission path is changed by commissioning or deployment of network devices or by network maintenance.